1. Field of the Invention
The invention concerns a method and devices for producing hydrogen by a plasma reforming operation.
With regard to the devices, the invention concerns hydrogen generators that are easily transportable and relatively inexpensive, adapted to produce substantially pure hydrogen for any purpose. The main use of such generators is to feed fuel cells installed on electrical cars or incorporated in generator sets.
With regard to the method, the invention concerns the production of a gas flow containing hydrogen for feeding a fuel cell operating at low temperature from a primary gas mixture comprising a combustible gas and steam and/or oxygen or air.
2. Background of the Invention
Hydrogen generators of the kinds with which the invention is concerned include a reaction chamber that is maintained at all times at a relatively high temperature suitable for reforming a primary gas mixture placed in a reactional state. A primary mixture of this kind comprises a combustible gas (hydrocarbon, alcohol, carbon monoxide, etc), oxygen and/or steam. In the reaction chamber the primary mixture undergoes endothermic or exothermic reforming that is more or less complete and in accordance with chemical equations (1) to (4) below, which equations describe reforming a stoichiometric mixture of methane, oxygen and steam. EQU CH.sub.4 +2H.sub.2 O.fwdarw.CO.sub.2 +4H.sub.2.fwdarw.strongly endothermic reaction (1) EQU CH.sub.4 +H.sub.2 O.fwdarw.CO+3H.sub.2.fwdarw.moderately endothermic reaction (2) EQU CO+H.sub.2 O.fwdarw.CO.sub.2 +H.sub.2.fwdarw.moderately exothermic reaction (3) EQU CH.sub.4 +O.sub.2.fwdarw.CO.sub.2 +2H.sub.2.fwdarw.strongly exothermic reaction (4)
Similar equations can be written in the case of reforming a primary gas mixture containing another hydrocarbon or an alcohol.
Such reforming converts the primary gas mixture into a secondary gas mixture formed of hydrogen and carbon dioxide as well as, usually, carbon monoxide and a residue of unconverted primary mixture.
Equations (2) and (3) above describe the intermediate steps generally involved in reforming in accordance with equation (1).
Thus carbon monoxide is generally produced during any operation of reforming a hydrocarbon or an alcohol. Carbon monoxide is known to act as a poison for one particularly interesting type of fuel cell, operating at low temperature and including a solid polymer electrolyte (proton exchange membrane (PEM) cells). Consequently, additional processing of the secondary mixture is essential for eliminating the carbon monoxide if the hydrogen obtained is to be usable directly in this type of fuel cell.
A distinction can be drawn between prior art reforming reaction chambers that use chemical catalysts and those which use a hot plasma to constitute a reactional medium. The documents commented on hereinafter describe three types of reaction chamber.
U.S. Pat. No. 4,981,676, granted in 1991 to Minet et al., describes a method for reforming a primary gas mixture of methane and steam. The reforming is carried out in an annular reaction chamber of great length and small diameter containing a catalyst material consisting of nickel-coated granules. The outside wall of the reaction chamber is a metal sheath and its inside wall is a membrane having relatively selective permeability for hydrogen, consisting of a porous ceramic, formed of a plurality of layers of decreasing thickness and porosity from the inside to the outside, with a thin catalytic metal layer on the outside. The membrane constitutes the wall of a chamber for collecting the hydrogen produced. The reaction chamber is heated externally by gas burners.
The advantages of the above method are selective in situ extraction of the nascent hydrogen produced. This shifts the point of thermodynamic equilibrium of conversions in accordance with equations (1), (2) and (3) in the direction of a more complete reaction and increases the rate of reforming, i.e. the rate of conversion of methane to hydrogen. The method has many disadvantages, which include: (1) limited application to steam reforming, (2) relatively fast aging and deterioration of the catalysts, requiring them to be replaced periodically, (3) production of a gas mixture formed of CO.sub.2, CO and H.sub.2. Consequently, a method of the above kind must include at least one additional stage for producing either substantially pure hydrogen or a gas mixture containing hydrogen suitable for a fuel cell operating at low temperature.
A substantially identical result obtained in the laboratory is described in an article by E. Kikuchi published by ELSEVIER, in Catalysis Today 25 (1995), pages 333-337. In the above article, the reactor again includes a sheath enclosing an annular space containing a standard catalyst material and a selective hydrogen extraction membrane. The membrane is a composite material formed of a thin (5 to 13 microns thick) layer of palladium or palladium-silver alloy deposited on a hollow porous ceramic support. In this case the conversions described by equations (1), (2) and (3) above are complete if the pressure is 9 bars and the temperature is 500.degree. C. This is because of the extraction of the nascent hydrogen produced, which shifts the thermodynamic equilibrium in the direction of more complete conversion. The drawbacks of this method are similar to those of the Minet patent.
Published European patent application No. 0 600 621 A1, filed by ROLLS-ROYCE in 1993, describes equipment for reforming a primary mixture of methane and steam which includes means for additional treatment of carbon monoxide contained in the secondary mixture produced. The equipment includes a reaction chamber including a significant mass of catalyst material adapted to assure endothermic conversion in accordance with equations (1) and (2). To this end the temperature in the reaction chamber is raised to a relatively high value by internal input of heat produced by partial oxidation of the methane by the exothermic reaction described by equation (4). Reactors containing a particular catalyst material assure additional slightly exothermic conversion of the carbon monoxide to carbon dioxide at relatively low temperature and in accordance with equation (3). This type of equipment, which is costly and bulky, is suitable for fixed industrial applications but not for transportable hydrogen generators.
Methods of producing hydrogen including hot plasma reforming of a mixture of hydrocarbons and steam are described in two further documents: (1) French patent application No. 94/11209 filed by Pompes Manu Entreprise et al. (called PME hereinafter) and (2) an article by O'Brien et al. of MIT published in an IEEE document in August 1996.
The PME reaction chamber uses a hot plasma produced by periodic electrical arcs sliding between two electrodes with a widening gap. The electrodes are connected to a permanent high voltage and are swept continuously by a relatively strong flow of the gas to be reformed. The sliding electrical arcs have a two-fold function, namely: ionizing the gases passing through the space between the electrodes and heating them to a high-level thermal equilibrium state (4,000 K to 10,000 K). The high-energy electrons created in this way stimulate the chemical reactivity of the gases concerned. On leaving the space between the electrodes these very hot gases, which are highly chemically reactive, are diluted in the remainder of the volume of the reaction chamber, which reduces their reactivity and their average temperature. Their final temperature is further reduced by virtue of the fact that the chamber is disposed in a hollow jacket through which The primary gas mixture flows and is thus preheated before it is injected into the chamber. Hydrogen and carbon monoxide are therefore produced at the same time. In operation, the PME reaction chamber uses a very large quantity of electricity to produce hydrogen. This makes this chamber entirely unsuitable for systems for producing electricity external use, whether transportable or not, especially as installing a prior art membrane that is selectively permeable to hydrogen in a PME chamber would not appear to be obvious to the skilled person.
The O'Brien reaction chamber uses a plasma torch producing a permanent electrical arc rotating between two coaxial electrodes connected to a high voltage supply. The functions and drawbacks of the O'Brien chamber are virtually the same as those of the electrical arc of the PME reaction chamber. Accordingly, this chamber is also unsuitable for producing a generator of substantially pure hydrogen.